1. Field of the Invention
The present invention relates to a current driver circuit for controlling output current.
2. Background Art
Hitherto, there is an automatic focus (AF) used in a cellular phone, a digital camera, and the like, to which a Voice Coil Motor (VCM) is applied.
The VCM is provided to move a lens by current driving. When the VCM is applied to a cellular phone or the like, from the viewpoint of battery operation, a wide range of voltages, for example, 2.3 V to 5.5 V is necessary as the source voltage.
The VCM is also requested to perform low-power-consumption and high-precision current value driving. Depending on an operation characteristic of the VCM, for example, the drive current of about 100 mA at the maximum is necessary.
In the case of low-voltage operation using, for example, a source voltage of 2.3 V, the voltage of a current drive terminal in a current driver circuit for controlling current to be supplied to the VCM is about 0.1 V to 0.2 V.
There is a conventional current driver circuit which detects output current and stores current value of output current to be adjusted (voltage value). The current driver circuit has a comparator for detecting that an output of a differential amplifier is zero, and adjusts an offset of the differential amplifier based on the detection result of the comparator (see Japanese Patent No. 4022059, for example).
As described above, the voltage of a current output terminal has to be low. Consequently, a sense resistor for detecting output current has considerably low resistance of, for example, about 0.5 Ω.
For this reason, for example, a detection value of the sense resistor is small, so that an offset held by an operational amplifier itself is unignorable. Further, in the case where an offset of the operational amplifier is large, an error in output current occurs, and there is a problem that it is difficult to set small output current.
As another conventional art, there is a method of detecting an offset voltage of a differential input of an operational amplifier directly by a comparator and, based on the detection result, adding offset correction current to the differential input of the operational amplifier to perform correction. In this case, as the operational amplifier, for example, a folded cascode operational amplifier is used. The offset correction current is passed to a folded differential pair of transistors from the outside.
However, in the conventional art, to directly detect the offset voltage of the operational amplifier, a high-precision comparator capable of detecting the difference of considerably small voltages is needed.
In the folded cascode operational amplifier, the offset adjustment is performed with a current value. Consequently, a current-type Digital to Analog Converter (DAC) has to be internally provided. In addition, to make the operational amplifier operate with high precision in a wide voltage range of 2.2 V to 5.5 V, the current source or the like of the operational amplifier has to have a cascode configuration.
As a result, the structure of the operational amplifier is complicated, the circuit scale is increased, and it causes a problem that power consumption increases.
Further, in the method of directly detecting the offset voltage of the operational amplifier, the correction cannot be made in the case where an output value of the DAC for offset adjustment changes due to power source/temperature fluctuations. Due to this, there is a problem that the influence of the fluctuation appears as an offset of the output current.